The suspects, aged between 24 and 47, were held on suspicion of conspiracy to steal from Barclays Bank and conspiracy to defraud UK banks. They are currently being questioned in police stations across London.

Known as a keyboard video mouse (KVM), the gadgets, which fit into the back of a machine, can cost as little as £10 and are widely available in computer stores or online.

The arrests are the latest in a series of suspected cyber crimes targeting banks in an attempt to steal millions of pounds.

Barclays reported the loss on April 5 but said no customers had taken losses to their accounts.

Officers discovered a device attached to branch computers that allowed a person to access private accounts. Police say one of the group’s members had gained access to the bank by claiming to be an IT engineer, and the man fitted the device to bank computers.